Organicus liquor regius

24 October 2010 - Physicial organic chemistry

Aqua regia is an inorganic mixture of nitric acid and hydrochloric acid well known to dissolve noble metals like gold. A group of researchers from the Georgia Institute of Technology have now come up with an organic solvent - a mixture of pyridine and thionyl chloride that also dissolves gold, palladium and silver but not platinum (Lin et al. DOI). A typical dissolution rate is 0.3 mol/m2/hr. The authors call their own invention of unprecedented scientific significance and kindly suggest a name for the new solvent: Organicus liquor regius.

As with aqua regia, to state that the metal is dissolved is misleading, rather gold is oxidized to the Au(III)Cl4- ion. Thionyl chloride itself is unable to do this but the charge-transfer complex with pyridine can. The researchers struggle to identify the counterion. They do find 4-chloropyridine and then pyridine oligomers which also explains the viscosity increase but no experimental evidence for reduced sulfur. Only tucked away in the supporting information, a plausible almost-balanced equation is presented.